The present invention relates to multi-color electro-optic media and to displays incorporating such media.
The term “electro-optic”, as applied to a material or a display, is used herein in its conventional meaning in the imaging art to refer to a material having first and second display states differing in at least one optical property, the material being changed from its first to its second display state by application of an electric field to the material. The optical property is typically color perceptible to the human eye.
The term “gray state” is used herein in its conventional meaning in the imaging art to refer to a state intermediate two extreme optical states of a pixel, and does not necessarily imply a black-white transition between these two extreme states. For example, several of the E Ink patents and published applications referred to below describe electrophoretic displays in which the extreme states are white and deep blue, so that an intermediate “gray state” would actually be pale blue. The terms “black” and “white” may be used hereinafter to refer to the two extreme optical states of a display, and should be understood as normally including extreme optical states which are not strictly black and white, for example the aforementioned white and dark blue states. The term “monochrome” may be used hereinafter to denote a drive scheme which only drives pixels to their two extreme optical states with no intervening gray states.
The terms “bistable” and “bistability” are used herein in their conventional meaning in the art to refer to displays comprising display elements having first and second display states differing in at least one optical property, and such that after any given element has been driven, by means of an addressing pulse of finite duration, to assume either its first or second display state, after the addressing pulse has terminated, that state will persist for at least several times, for example at least four times, the minimum duration of the addressing pulse required to change the state of the display element. It is shown in U.S. Pat. No. 7,170,670 that some particle-based electrophoretic displays capable of gray scale are stable not only in their extreme black and white states but also in their intermediate gray states, and the same is true of some other types of electro-optic displays. This type of display is properly called “multi-stable” rather than bistable, although for convenience the term “bistable” may be used herein to cover both bistable and multi-stable displays.
Several types of electro-optic displays are known. One type of electro-optic display is a rotating bichromal member type as described, for example, in U.S. Pat. Nos. 5,808,783; 5,777,782; 5,760,761; 6,054,071 6,055,091; 6,097,531; 6,128,124; 6,137,467; and 6,147,791 (although this type of display is often referred to as a “rotating bichromal ball” display, the term “rotating bichromal member” is preferred as more accurate since in some of the patents mentioned above the rotating members are not spherical). Such a display uses a large number of small bodies (typically spherical or cylindrical) which have two or more sections with differing optical characteristics, and an internal dipole. These bodies are suspended within liquid-filled vacuoles within a matrix, the vacuoles being filled with liquid so that the bodies are free to rotate. The appearance of the display is changed by applying an electric field thereto, thus rotating the bodies to various positions and varying which of the sections of the bodies is seen through a viewing surface. This type of electro-optic medium is typically bistable.
Another type of electro-optic display uses an electrochromic medium, for example an electrochromic medium in the form of a nanochromic film comprising an electrode formed at least in part from a semi-conducting metal oxide and a plurality of dye molecules capable of reversible color change attached to the electrode; see, for example O'Regan, B., et al., Nature 1991, 353, 737; and Wood, D., Information Display, 18(3), 24 (March 2002). See also Bach, U., et al., Adv. Mater., 2002, 14(11), 845. Nanochromic films of this type are also described, for example, in U.S. Pat. Nos. 6,301,038; 6,870,657; and 6,950,220. This type of medium is also typically bistable.
Another type of electro-optic display is an electro-wetting display developed by Philips and described in Hayes, R. A., et al., “Video-Speed Electronic Paper Based on Electrowetting”, Nature, 425, 383-385 (2003). It is shown in U.S. Pat. No. 7,420,549 that such electro-wetting displays can be made bistable.
Particle-based electrophoretic displays, in which a plurality of charged particles move through a fluid under the influence of an electric field, have been the subject of intense research and development for a number of years. Electrophoretic displays can have attributes of good brightness and contrast, wide viewing angles, state bistability, and low power consumption when compared with liquid crystal displays. Nevertheless, problems with the long-term image quality of these displays have prevented their widespread usage. For example, particles that make up electrophoretic displays tend to settle, resulting in inadequate service-life for these displays.
As noted above, electrophoretic media require the presence of a fluid. In most prior art electrophoretic media, this fluid is a liquid, but electrophoretic media can be produced using gaseous fluids; see, for example, Kitamura, T., et al., “Electrical toner movement for electronic paper-like display”, IDW Japan, 2001, Paper HCS1-1, and Yamaguchi, Y, et al., “Toner display using insulative particles charged triboelectrically”, IDW Japan, 2001, Paper AMD4-4). See also U.S. Pat. Nos. 7,321,459 and 7,236,291. Such gas-based electrophoretic media appear to be susceptible to the same types of problems due to particle settling as liquid-based electrophoretic media, when the media are used in an orientation which permits such settling, for example in a sign where the medium is disposed in a vertical plane. Indeed, particle settling appears to be a more serious problem in gas-based electrophoretic media than in liquid-based ones, since the lower viscosity of gaseous suspending fluids as compared with liquid ones allows more rapid settling of the electrophoretic particles.
Numerous patents and applications assigned to or in the names of the Massachusetts Institute of Technology (MIT) and E Ink Corporation describe various technologies used in encapsulated electrophoretic and other electro-optic media. Such encapsulated media comprise numerous small capsules, each of which itself comprises an internal phase containing electrophoretically-mobile particles in a fluid medium, and a capsule wall surrounding the internal phase. Typically, the capsules are themselves held within a polymeric binder to form a coherent layer positioned between two electrodes. The technologies described in the these patents and applications include:                (a) Electrophoretic particles, fluids and fluid additives; see for example U.S. Pat. Nos. 7,002,728; and 7,679,814;        (b) Capsules, binders and encapsulation processes; see for example U.S. Pat. Nos. 6,922,276 and; 7,411,719;        (c) Films and sub-assemblies containing electro-optic materials; see for example U.S. Pat. Nos. 6,982,178; and 7,839,564;        (d) Backplanes, adhesive layers and other auxiliary layers and methods used in displays; see for example U.S. Pat. Nos. 7,116,318; and 7,535,624;        (e) Color formation and color adjustment; see for example U.S. Pat. Nos. 6,017,584; 6,664,944; 6,864,875; 7,075,502; 7,167,155; 7,667,684; and 7,791,789; and U.S. Patent Applications Publication Nos. 2004/0263947; 2007/0109219; 2007/0223079; 2008/0023332; 2008/0043318; 2008/0048970; 2008/0211764; 2009/0004442; 2009/0225398; 2009/0237776; 2010/0103502; 2010/0156780; and 2010/0225995;        (f) Methods for driving displays; see for example U.S. Pat. Nos. 7,012,600; and 7,453,445;        (g) Applications of displays; see for example U.S. Pat. No. 7,312,784; and U.S. Patent Application Publication No. 2006/0279527.        
Many of the aforementioned patents and applications recognize that the walls surrounding the discrete microcapsules in an encapsulated electrophoretic medium could be replaced by a continuous phase, thus producing a so-called polymer-dispersed electrophoretic display, in which the electrophoretic medium comprises a plurality of discrete droplets of an electrophoretic fluid and a continuous phase of a polymeric material, and that the discrete droplets of electrophoretic fluid within such a polymer-dispersed electrophoretic display may be regarded as capsules or microcapsules even though no discrete capsule membrane is associated with each individual droplet; see for example, U.S. Pat. No. 6,866,760. Accordingly, for purposes of the present application, such polymer-dispersed electrophoretic media are regarded as sub-species of encapsulated electrophoretic media.
A related type of electrophoretic display is a so-called “microcell electrophoretic display”. In a microcell electrophoretic display, the charged particles and the fluid are not encapsulated within microcapsules but instead are retained within a plurality of cavities formed within a carrier medium, typically a polymeric film. See, for example, U.S. Pat. Nos. 6,672,921 and 6,788,449, both assigned to Sipix Imaging, Inc. Hereinafter, the term “microcavity electrophoretic display” may be used to cover both encapsulated (including polymer-dispersed) and microcell electrophoretic displays.
Although electrophoretic media are often opaque (since, for example, in many electrophoretic media, the particles substantially block transmission of visible light through the display) and operate in a reflective mode, many electrophoretic displays can be made to operate in a so-called “shutter mode” in which one display state is substantially opaque and one is light-transmissive. See, for example, U.S. Pat. Nos. 5,872,552; 6,130,774; 6,144,361; 6,172,798; 6,271,823; 6,225,971; and 6,184,856. Dielectrophoretic displays, which are similar to electrophoretic displays but rely upon variations in electric field strength, can operate in a similar mode; see U.S. Pat. No. 4,418,346. Electrophoretic media operating in shutter mode may be useful in multi-layer structures for full color displays; in such structures, at least one layer adjacent the viewing surface of the display operates in shutter mode to expose or conceal a second layer more distant from the viewing surface.
As already indicated, an encapsulated or microcell electrophoretic display typically does not suffer from the clustering and settling failure mode of traditional electrophoretic devices and provides further advantages, such as the ability to print or coat the display on a wide variety of flexible and rigid substrates. (Use of the word “printing” is intended to include all forms of printing and coating, including, but without limitation: pre-metered coatings such as patch die coating, slot or extrusion coating, slide or cascade coating, curtain coating; roll coating such as knife over roll coating, forward and reverse roll coating; gravure coating; dip coating; spray coating; meniscus coating; spin coating; brush coating; air knife coating; silk screen printing processes; electrostatic printing processes; thermal printing processes; ink jet printing processes; electrophoretic deposition (See U.S. Pat. No. 7,339,715); and other similar techniques.) Thus, the resulting display can be flexible. Further, because the display medium can be printed (using a variety of methods), the display itself can be made inexpensively.
There is today an increasing demand for color in all displays. Users familiar with color televisions, color computer displays and color displays on cellular telephones and other portable electronic devices may regard monochrome display as lacking something in visual appeal even in applications such as electronic book readers, where the display aims to reproduce the look of a printed book, most of which are still printed in monochrome.
In conventional printing, full color images of high quality are formed by providing sub-images in each of three subtractive primary colors, typically cyan, magenta and yellow (“CMY”) (black may be included as a four primary in a “CMYK” system) that are overlaid (i.e., more than one color can be present at any point on the page) in such a way that light is filtered through each sub-image before being reflected back from the underlying white paper to the viewer. (Thus, a so-called “four color”, CMYK system is in reality a five-color system; the white color of the underlying paper is part of the color formation system, as is readily appreciated from the fact that this white color appears where no ink whatever is present.) In this arrangement of three or four overlaid sub-images, no area of the printed paper absorbs light unnecessarily, and thus an image of maximum brightness is obtained.
Prior art electrophoretic and similar electro-optic displays have typically relied upon the use of reflective (light-scattering) pigments. Since no substantial amount of light passes through a layer of such pigment, it is not possible to overlay sub-images of differing colors, and in a color display it is necessary to resort to “color area sharing” to render a palette of colors. For example, multiple different sub-regions of the display may be provided with electrophoretic media capable of displaying different colors, for example red, green and blue. (Note that since there is no overlaying of sub-images of different colors, this type of display typically uses additive primaries rather than subtractive primaries.) Alternatively, a monochrome medium can be used and a color filter array provided so that specific pixels can reflect specific primary colors. Either approach suffers, however, from the problem that only a fraction of the area of the display is available for reflection of each primary color, which adversely affects the brightness of the image available. Hence, to improve the brightness of a color reflective display, it is desirable to provide a display which can display any desired color at any pixel of the display, and thereby to maximize the amount of light reflected to a viewer.
Multilayer, stacked electro-optic displays using are known in the prior art. In such displays, ambient light passes through sub-images in each of the three subtractive primary colors, in a manner analogous to conventional color printing. U.S. Pat. No. 6,727,873 describes a stacked electrophoretic display in which three layers of switchable cells are placed over a reflective background. Similar displays are known in which pigments are moved laterally; see for example International Application Publication No. WO 2008/065605) or, pigments in microcavities are moved using a combination of vertical and lateral motion. For a review of such displays, see J. Heikenfeld, P., et al., Journal of the SID, 19(2), 2011, pp. 129-156. In these prior art displays, each pixel of each layer must be capable of being driven independently so as to concentrate or disperse the pigment particles on a pixel-by-pixel basis. This requires three separate pair of electrodes, each of which typically comprises an active matrix backplane having a matrix of thin-film transistors, and an opposed continuous counter-electrode. Two of the active matrix backplanes must be as transparent as possible, as must be each counter-electrode. This approach suffers from the severe disadvantages of the high cost of manufacturing such a complex arrangement of electrodes, and from the fact that in the present state of the art it is difficult to provide an adequately transparent backplane, especially as the white state of the display requires that light pass through several layers of such transparent electrodes; in practice, the light losses in the electrodes have a severe adverse effect on the brightness of the image produced by the display.
Those skilled in the imaging art know that it is necessary to provide independent addressing of each primary color in order to render a full-color image. This is illustrated graphically in FIG. 1 of the accompanying drawings, which shows a “color cube” in which the vertices correspond to white, the three subtractive primary colors (yellow, magenta, and cyan), the three additive primary colors (red, green and blue) and black. As shown by the arrows, any point inside or on the surface of the color cube can be defined by three (orthogonal) co-ordinates, namely, the distances along the white-yellow axis, the white-magenta axis, and the white-cyan axis. These distances correspond to different optical densities in the subtractive primary colors, ranging from zero (i.e., white) to about 2 (corresponding 99% absorption of light of the corresponding additive primary spectral range). The number of discretely addressed independent states required to render the full color gamut of a display is the number of yellow states plus the number of magenta states plus the number of cyan states. The number of colors that can be rendered, however, is the product of these three numbers. Thus, for example, a display may be chosen to render 2 yellow states (since the human visual system is relatively insensitive to spatial variation in blue light, the absence of which corresponds to the yellow subtractive primary color) and 24=16 states of each of magenta and cyan. The waveform driving the display would be required to render 34 different states in total, but would be able to address 29=512 different colors.
In one aspect, this invention provides a color display in which a single pair of electrodes are used to address independently more than one layer of electrophoretic or similar electro-optic material. Such a color display can provide independent, or at least partially independent addressing of more than one primary color using a single pair or single set of electrodes (for example, the single set of electrodes can be an active matrix backplane and a single continuous counter-electrode. At least one of the layers of electrophoretic or similar material may operate in shutter mode (as defined above).
Shutter mode electrophoretic displays can be used as light modulators. Light modulators represent a potentially important market for electro-optic media. As the energy performance of buildings and vehicles becomes increasingly important, electro-optic media can be used as coatings on windows (including skylights and sunroofs) to enable the proportion of incident radiation transmitted through the windows to be electronically controlled by varying the optical state of the electro-optic media. Effective implementation of such “variable-transmissivity” (“VT”) technology in buildings is expected to provide (1) reduction of unwanted heating effects during hot weather, thus reducing the amount of energy needed for cooling, the size of air conditioning plants, and peak electricity demand; (2) increased use of natural daylight, thus reducing energy used for lighting and peak electricity demand; and (3) increased occupant comfort by increasing both thermal and visual comfort. Even greater benefits would be expected to accrue in an automobile, where the ratio of glazed surface to enclosed volume is significantly larger than in a typical building. Specifically, effective implementation of VT technology in automobiles is expected to provide not only the aforementioned benefits but also (1) increased motoring safety, (2) reduced glare, (3) enhanced mirror performance (by using an electro-optic coating on the mirror), and (4) increased ability to use heads-up displays. Other potential applications of VT technology include privacy glass and glare-guards in electronic devices.
VT media capable of being used as VT windows have been demonstrated and are described in the patent literature; see, for example, U.S. Pat. No. 7,327,511; and U.S. Patent Applications Publication Nos. 2006/0038772; 2007/0146310; and 2008/0130092. However, there are certain remaining problems in such VT media. Firstly, it is difficult to achieve in the same medium acceptable levels of image stability (i.e., stable transmission) and haze. In practice, VT windows need high image stability of the order of several hours, since users do not wish to have their windows adjusting their transmission levels every few minutes. As discussed above (see the aforementioned U.S. Pat. No. 7,170,670), the image stability of electrophoretic media can be increased by dissolving or dispersing a high molecular weight polymer in the fluid in the fluid in which the electrophoretic particles (typically carbon black particles) are dispersed; the aforementioned 2007/0146310 recommends a polystyrene dispersed in a mixture of limonene and a partially hydrogenated terphenyl as a fluid for used in VT electrophoretic media. The effect of adding the polymer to the fluid is to increase the flocculation of the electrophoretic particles once the particles are aggregated together, and thus to increase the image stability. Unfortunately, the polymer also serves to increase particle flocculation even when the electrophoretic particles are intended to be dispersed throughout the fluid, and the resultant increase in particle aggregation and thus effective particle size, substantially increases the optical haze of the medium; the particle size of the aggregated particles is large enough to cause substantial scattering of visible light, even though the individual particles themselves are sufficiently small that they will cause little scattering. The light scattering responsible for haze depends upon both the particle size and the difference in refractive index between the electrophoretic particles and the surrounding fluid. To date, no black pigment has been identified which has a refractive index close enough to that of the fluids typically used in electrophoretic media (or close enough to that of the polymeric phase which typically surrounds the fluid, as discussed above) to reduce haze to an acceptable level, and no mechanism for increasing image stability to the degree considered desirable for commercial sale has been identified which does not increase haze to an undesirable level.
Another problem with prior art VT electrophoretic media (and similar electro-optic media such as electrochromic media) is their inability to vary hue; in other words, the colors capable of being displayed by such media fall on a line between their endpoint colors (a transparent state being regarded as a “color” for purposes of the present application), and the media do not have a color gamut volume. For example, the colors obtainable from the VT media described in the aforementioned U.S. Pat. No. 7,327,511; and U.S. Patent Applications Publication Nos. 2006/0038772; 2007/0146310; and 2008/0130092 vary from black to clear, while electrochromic media typically vary from blue-purple to clear. (Provision of color in VT media may be useful either in enabling the light within a room equipped with VT windows to be varied, or in enabling the use of VT as one layer in a multi-layer display, as discussed in detail below.) Neither type of media can produce additional colors without the addition of a color filter array, and the use of a multi-pixel drive method, typically using a passive or active matrix backplane. Such a backplane inevitably reduces optical transmission through the VT medium and is far more expensive than the simple electrode used in a single pixel VT display
Accordingly, there is still a need for a VT medium which can provide the high image stability desirable in commercial VT displays in combination with low haze. There is also still a need for VT media which can provide a substantial color gamut. In one aspect, the present invention seeks to provide solutions to both these problems.